1. Field of the Invention
This invention relates to a variable impedance circuit device whose impedance can be varied by means of electric control, and more particularly it pertains to such a circuit device which is so designed that the impedance thereof can be varied over a wide range.
2. Description of the Prior Art
In order to have a better understanding of the present invention, reference will first be made to FIG. 1 of the accompanying drawings, which illustrates a conventional variable impedance circuit which comprises diode-connected transistors Q.sub.1 and Q.sub.2. The illustrated circuit arrangement includes an input terminal 1, a terminal 2 to which is applied a reference voltage, a power supply terminal 3, and current source circuits 5 and 6. The transistors Q.sub.1 and Q.sub.2 have the emitters thereof connected together and to a variable current source 4. With such a circuit arrangement, by changing a control voltage applied to the variable current source 4 to cause a variable current to flow through each of the diode-connected transistors Q.sub.1 and Q.sub.2, a variable impedance element is provided between the terminals 1 and 2. Indicated at 7 is a by-pass capacitor. More specifically, in the circuit arrangement shown in FIG. 1, equal bias currents I.sub.0 flow through the current sources 5 and 6 which are connected to the collectors of the transistors Q.sub.1 and Q.sub.2 respectively so that a current 2I.sub.0 flows through the current source 4 connected to the emitters of these transistors. When a signal source is connected to the terminal 1, an input current i.sub.1 flows in the circuit, and an output current i.sub.2 flows out of it. Under such a condition, the relationship i.sub.l =i.sub.2 holds true. Instantaneous input voltage V.sub.R applied to the terminal 1 is equal to the difference between base-emitter voltages V.sub.BE1 and V.sub.BE2 of the transistors Q.sub.1 and Q.sub.2 respectively. Assuming that a current i.sub.R is generated on the basis of the instantaneous input voltage V.sub.R, then the relationship i.sub.R =i.sub.1 =i.sub.2 holds true. Thus, the instantaneous input voltage V.sub.R is given by the following expression: ##EQU1## where V.sub.T =KT/q; I.sub.S is saturation current; K is Boltzmann's constant; T is absolute temperature; and q is electron charge. As will be seen from Equation (1), the instantaneous input voltage V.sub.R depends on the input current i.sub.1 flowing in the terminal 1, and the factor thereof can be controlled by means of the equal bias currents I.sub.0. In a small signal range, the impedance R between the terminals 1 and 2 can be determined by differentiating Equation (1) in terms of i.sub.R and seeking the gradient of a tangential line with i.sub.R =0, and given as follows: ##EQU2## From Equation (2), it will be seen that the impedance R can be proportionally controlled on the basis of the bias currents I.sub.0 which are variable d.c. currents.
However, the foregoing prior-art circuit arrangement is disadvantageous in that since equally controlled variable d.c. currents are applied to the diode-connected transistors Q.sub.1 and Q.sub.2 to cause the bias currents to flow therethrough, there is the tendency that if the equilibrium between the variable d.c. voltages is disturbed, then a current is flown in or drawn in through the terminal 1 or 2 so that the d.c. bias for each of the transistors is suddenly changed, thus imparting distortion to the input signal i.sub.1 which is inputted through the terminal 1. Thus, if the aforementioned prior-art variable impedance circuit arrangement is employed as a noise suppressor circuit, gain control circuit or the like for a radio receiver, then pop noise will disadvantageously be caused due to the sudden d.c. bias changes.
Another disadvantage is such that in the case where the variable current source 4 is comprised of a transistor, it is not possible to flow the control current I.sub.0 by the use of a control voltage lower than about 0.6 V, i.e., the base-emitter voltage of the transistor. In other words, in such a case, the impedance of the circuit shown in FIG. 1 cannot be varied with a control voltage in the range of 0 to 0.6 V or less. Obviously, in the case where the bias circuit for the transistor source 4 is comprised of two diodes connected in series with each other, the impedance cannot be varied with a control voltage lower than about 1.2 V. Thus, the prior-art circuit arrangement shown in FIG. 1 has such a drawback that in the case where the power source voltage V.sub.CC is less than 1.8 V, a sufficient quantity of the control current I.sub.0 cannot be flown with a control voltage in the range of 0 to 1.2 V so that the impedance of the circuit can only be varied over a narrow range.